Global warming refers to the theory that the Earth’s atmosphere, specifically, the troposphere or lower atmosphere, is retaining more heat because of an increase in the amount of greenhouse gases.
These gases include carbon dioxide, methane, and nitrous oxide, and together they prevent about 30 percent of the heat that leaves the Earth’s surface from escaping into space.
The amount of these gases in the atmosphere has increased since the Industrial Revolution began more than 200 years ago.
As a result, more heat is retained on Earth, causing temperatures to rise worldwide.
It appears that the amount of carbon dioxide in the atmosphere has increased by 8 percent since the 1970s, the direct result of humans burning fossil fuels such as oil, gas, and coal, which give off large amounts of carbon dioxide.
Some scientists predict that rising levels of carbon dioxide in the atmosphere will eventually cause average temperatures to increase by several degrees.
If this happens, polar ice would begin to melt, causing ocean levels to rise and threaten coastal cities.
Food crops also would be severely affected by temperature change.
But other scientists disagree, they think that the increases in temperature over the past 200 years are part of a natural cycle of temperature change, even though they agree that human activity is increasing the amount of greenhouse gases.